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brown, which would indicate too large an excess of the zinc oxide) and the
supernatant liquid appears colourless.1 The precipitate settles rapidly if
the flask is held inclined in a suitable stand.

When the precipitate has deposited, 10 c.c. of the permanganate solu-
tion are added, the liquid shaken, the precipitate allowed to settle and, if
the supernatant liquid is colourless, a further 10 c.c. of permanganate are
added, this procedure being continued until the liquid contains excess of
the reagent. The excess of permanganate is titrated witn the sodium
arsenite solution, which is added gradually and with shaking and with an
interval after each addition until the liquid is decolorised. From the
quantity of permanganate used, less that corresponding with the arsenite
solution added (i c.c. of arsenite = about 0-5 c.c. of permanganate), the
amount of permanganate required to oxidise the manganese completely
is calculated approximately.

Titration. For the actual titration of the manganese, an aliquot part
of the liquid equal to that used in the preliminary test is oxidised with
hydrogen peroxide, boiled, diluted with boiling water to 600-700 c.c. and
zinc oxide added to precipitate the iron.

While the precipitate is settling, the volume of permanganate found
necessary plus 3-4 c.c. is introduced into a beaker and then rapidly poured
into the flask, the latter being shaken and the beaker rinsed out with water
into the flask. When the precipitate has settled again, the excess of per-
manganate added is determined by titration with the sodium arsenite as
in the preliminary test. Next, in order to determine the true titre of the
arsenite with respect to the permanganate under the exact conditions used,
a further volume of 5 c.c. of the permanganate is added and the liquid
again decolorised by the arsenite solution. From these data a simple
calculation gives the amount of permanganate required for the complete
oxidation of the manganese, and hence the amount of the latter.

It is always advisable to carry out control determinations on different
aliquot parts of the solution.

EXAMPLE, i c.c. of the permanganate solution is found to correspond
with 0-00165 gram of manganese. In the actual test, 35 c.c. of permanganate
were added and the excess required 8 c.c. of arsenite solution, of which 9 c.c,
correspond with 5 c.c. of permanganate. Since

9:5 = 8: 4-44,

the amount of permanganate reduced will be 35-4-44 = 30-56 c.c. and 30-56
X 0-00165  amount of manganese in the aliquot part of the solution taken.

Volhard's method is used more especially in laboratories where estimations
of manganese are made regularly ; it is fairly exact and, the solutions being
ready, fairly rapid. It cannot be used directly in presence of chromium, van-
adium and cobalt, these also reducing the permanganate (for the determination
of manganese in presence of chromium, see p. 185). Further, to steels contain-
ing large proportions of nickel the method is not applicable, since the liquid
remains greenish-yellow and the exact end of the reaction cannot be ascertained
(for the determination of manganese in presence of large quantities of nickel, see
p. 186). Many authors, instead of adding excess of permanganate and then

1 It is necessary to add a slight excess of zinc oxide, but not a large excess, -which,
may be harmful. Chim. Min. e